Lewis acids transition metal-based

Acidic chloroaluminate ionic liquids have already been described as both solvents and catalysts for reactions conventionally catalyzed by AICI3, such as catalytic Friedel-Crafts alkylation [35] or stoichiometric Friedel-Crafts acylation [36], in Section 5.1. In a very similar manner, Lewis-acidic transition metal complexes can form complex anions by reaction with organic halide salts. Seddon and co-workers, for example, patented a Friedel-Crafts acylation process based on an acidic chloro-ferrate ionic liquid catalyst [37]. 

Catalytic asymmetric allylation of aldehydes and ketones with allylsilanes can be achieved by using chiral Lewis acids, transition metal complexes, and Lewis bases. In recent years, much attention has been paid for the chiral Lewis base-catalyzed system using allyltrichlorosilanes. Advances in catalytic asymmetric carbonyl allylation have been described in detail in recent reviews.116,117,117a... 

An interesting alternative to stereogenic carbon-based auxiliaries is the ap-phcation of chiral Lewis acidic transition metal fragments (Scheme 93) [217-... 

A wide variety of Arrhenius acids, Lewis acids, transition metal compounds, and Lewis bases have been used to cleave the Si-0 bond in TBDMSOR ethers for parent alcohol regeneration [ 148]. It does not appear, however, that the use of a nonionic base in such transformations has been reported. 

The addition of cyanotrialkylsilanes to carbonyl compounds, and especially the development of catalysts for this transformation continues to attract considerable attention owing to the role of cyanohydrin trialkysilyl ethers and cyanohydrins as versatile intermediates in organic synthesis [149]. A variety of catalysts has been developed for such reactions including Lewis acids, transition metal complexes, 18-crown-6 complexes of alkali metals, tetracyanoethylene, Lewis bases, and alkali earth bases. Trimethylcyanation of ketones using Lewis bases does not seem to have been mentioned in the literature. 

This chapter documents advances in the use of catalysis for regioselective transformations of carbohydrate derivatives. For this discussion, the catalysts are grouped into broad structural or functional classes (Lewis bases, Brpnsted acids, Lewis acids, transition metals). Processes that make use of enzyme catalysis (chemoenzymatic methods or metaboUc engineering) are not discussed here [8-10]. Emphasis is placed oti recent results that were not discussed in previous reviews on this and related topics [11—13]. 

Transition-metal-based Lewis acids such as molybdenum and tungsten nitro-syl complexes have been found to be active catalysts [49]. The ruthenium-based catalyst 50 (Figure 3.6) is very effective for cycloadditions with aldehyde- and ketone-bearing dienophiles but is ineffective for a,)S-unsaturated esters [50]. It can be handled without special precautions since it is stable in air, does not require dry solvents and does not cause polymerization of the substrates. Nitromethane was the most convenient organic solvent the reaction can also be carried out in water. 

A variety of other transition-metal-based Lewis acids are used in organic transformations. Some of them are water tolerant and promising as sources of highly functionalized Lewis-acid catalysts. 

General acid-base catalysis is often the controlling factor in many mechanisms and acts via highly efficient and sometimes intricate proton transfers. Whereas log K versus pH profiles for conventional acid-base catalyzed chemical processes pass through a minimum around pH 7.0, this pH value for enzyme reactions is often the maximum. In enzymes, the transition metal ion Zn2+ usually displays the classic role of a Lewis acid, however, metal-free examples such as lysozyme are known too. Good examples of acid-base catalysis are the mechanisms of carbonic anhydrase II and both heme- and vanadium-containing haloperoxidase. 

Because of its large size, the electron cloud of FT is easily polarizable, and therefore, H is a soft base. Consequently, complexes with transition metals are usually formed in which the metals are soft Lewis acids. The metals are usually in low oxidation states, which makes them softer acids than the same metals in higher oxidation states. Typical among these compounds are Fe(CO)4H2, Re(CO)(P(C6H5)3)3H, and Mn(CO)5H. 

Not all complexes are purely electrostatic. In fact, many metal complexes in biological systems have covalent interactions as well. In these cases, the ligand donates a pair of electrons (acting as a Lewis base) to the metal, which functions as a Lewis acid. Therefore, metals can be evaluated based on their abilities to accept electron pairs. Alkali metal ions (Na+, K+) and alkaline earth metals (Mg2+, Ca2+) tend to not form stable complexes with Lewis base ligands. Transition metal ions, particularly those with vacant ri-orbitals, will form more stable complexes with Lewis base-acting ligands. 

Transition metal-based Lewis acid, base, and ambiphilic catalysis in syntheses of O- and A-heterocycles 03YGK425. 

A small number of enantiomerically pure Lewis acid catalysts have been investigated in an effort to develop a catalytic asymmetric process. Initial work in this area was carried out by Narasaka and coworkers using the titanium complex derived from diol (8.216) in the cycloaddition of electron-deficient oxazolidinones such as (8.217) with ketene dithioacetal (8.218), alkenyl sulfides and alkynyl sulfides. Cyclic alkenes can be used in this reaction and up to 73% ee has been obtained in the [2- -2] cycloaddition ofthioacetylene (8.220) and derivatives with2-methoxycarbonyl-2-cyclopenten-l-one (8.221) usingthe copper catalyst generated with bis-pyridine (8.222). Furthermore, up to 99% ee has been obtained in the [2-1-2] cycloaddition of norbornene with alkynyl esters using rhodium/Hs-BINAP catalysts. This reaction is not restricted to the use of transition metal-based Lewis... 

It is difficult to summarize all the progress of the homogeneous catalytic aldol reaction due to the space limitations. With representative or selected examples, this article focuses on those efforts in homogeneous catalysis and will be discussed with selected examples according to the different types of catalysts il) transition metals based Lewis acids, such as palladium, copper, iron, etc (2)... 

Several transition metal-based Lewis acid catalysts such as FeCls, CrCls, SnCU, or A1(03S0Ci2H25 )s were shown to be highly effective for aza-type Michael additions of amines such as indoles and pyrroles to a,/ -unsaturated compounds in aqueous solution. Addition of thiols to a,/ -unsaturated ketones in water was also catalyzed by SDS or /3-cyclodextrin. There is also a single report on the stereoselectivity of base-catalyzed Michael additions of thiols to nitro-olefins, where moderate diastereoselectivities were obtained (Scheme 8.13). 

Xu, L.-W., Li, L. and Xia, C. G., Transition-metal-based Lewis acid catalysis of aza-type Michael additions of amines to a,/ -unsaturated electrophiles in water, Helv. Chim. Acta, 2004, 87,1522-1526. 

The first observable transition-metal based Lewis acid that is optically active with metal-centred chirality has been claimed for [CpRe(NO)(PPh3)]PF5. Reactions of this substrate with Lewis bases has also been described. ... 

Not only the highly Lewis acidic early transition metal-based polymerization catalysts suffer from poisoning by coordination of functional groups. Even in late transition metal-based complexes, the possible o-coordination in certain functional groups has a negative impact on polymerization reactions. The prominent example here is the still ongoing search for active acrylonitrile (AN) copolymerization catalysts. This reaction can serve as an ideal example to illustrate the challenges in late transition metal-catalyzed insertion polymerizations with polar functionalized comonomers. The metal-mediated copolymerization of AN has numerous appearances in literature however, in most cases, the reaction mechanism seems to be of ionic or radical nature. 

The cycloaddition of carbon-heteroatom imsaturated bonds to alkynes is a versatile method for the construction of six-membered heterocyclic molecular frameworks such as pyridine and pyran, which are widely distributed in natural products and pharmaceutical drugs. In the past few decades, transition-metal-catalyzed cycloaddition has emerged as apowerful method for the synthesis of structurally diverse heterocyclic compounds [1], Nickel complexes are the most significant transition-metal-based catalysts because as opposed to divalent nickel, which shows ir-Lewis acid behavior toward carbon-carbon unsaturated bonds, zero-valent nickel shows nucleophilic behavior toward carbon-heteroatom unsaturated bonds, thereby facilitating the formation of heteronickelacycles, which are key intermediates for heterocycle synthesis... 

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